How to trace back an unknown production temperature of biochar from chemical characterization methods in a feedstock independent way

Highlights

• 24 biochar samples from 12 different feedstocks were characterised using five different chemical characterization methods.

• Five feedstock independent indicators were identified based on the principal component analysis.

• The highest treatment temperature was modelled using three feedstock-independent indicators.

• The multilinear model and auxiliary correlations were positively validated with external datasets.

Abstract

Besides the feedstock composition, the highest treatment temperature (HTT) in pyrolysis is one of the key production parameters. The latter determines the feedstock’s carbonization extent, which influences physicochemical properties of the resulting biochar, and in consequence its performance in industrial and agricultural applications. The actual HTT of biomass is difficult to measure in a reliable manner in many large-scale pyrolysis units (e.g., rotary kilns). Therefore, producers and end-users often rely on unreliable or biased information regarding this key production parameter that affects biochar quality. Data from indirect chemical assessment methods of biochar’s carbonization extent correlate well with the highest treatment temperature. Therefore, this study demonstrates that the HTT can be accurately assessed posteriori and feedstock-independently via a simple-to-use model based on biochar characteristics related to the carbonization extent. For that purpose, 24 contrasting biochars from 12 different feedstocks produced in the most common production temperature range of 350−700 °C were analysed using 5 different established biochar chemical characterization methods. Then, experimental data was used to establish a multilinear regression model capable of correlating the HTT, which was successfully validated for external datasets. The correlation accuracy for biochars of various origin (lignocellulosic, manure) was satisfactorily high (R²adj. = 0.853, RSME =47 °C). The obtained correlation proved that the HTT can be predicted feedstock independently with the use of basic input data. It also provides a quick, simple, and reliable tool to verify the HTT of a given biochar.

Introduction

Biochar is the solid, carbon-rich product obtained through pyrolysis of biomass, typically being forestry and agricultural residues or wastes [1]. The production and application of biochar is increasingly gaining interest worldwide. The properties of biochar mainly dictate its possible applications and strongly depend on the carbonization level, which is governed by the feedstock and pyrolysis process conditions used during its production [2]. Several studies have shown a significant correlation between the HTT and biochar’s composition (e.g., carbon content, H/C and O/C molar ratio) as well as its structural properties (e.g., BET surface area, micropore volume and surface functionality) [3,4]. Although these features generally correlate with the HTT, significant scattering in the correlations remains due to the feedstock dependence of mentioned parameters.

The effect of feedstock-dependent features on the biochar’s structural organisation is harder to predict and to control than the influence of production-dependent features, such as the HTT. In laboratory-scale biochar production, the HTT can theoretically be measured adequately, if multiple thermocouple are in place at various positions. Yet, this is however not always the case, as betimes a set reactor temperature is reported, rather than an actually measured temperature inside a biomass bed. Moreover, the HTT during industrial scale biochar production can vary from the one put forth by the producers. Indeed, the actual production temperature not always reaches the desired pyrolysis temperature along with the HTT (i.e. in between batches or in continuous pyrolysis reactors). The variation in the moisture content of the used feedstock or temperature gradient inside the reactor can be identified as main contributors for that discrepancy. The endothermicity/exothermicity of the pyrolysis reactions (i.e. its endo or exothermal nature) which can shift the actual HTT in case of conversion of large particles, also contributes to that discrepancy. Moreover, the biochar HTT of different suppliers provided as “production temperature” can also be measured ambiguously (ex-bed, in-bed, etc.) or might be not measured at all (i.e. in simple kilns). Finally, in some instances, a biochar applier may be offered biochar whose production history details not or incompletely known. Since the properties of biochar can be strongly feedstock-dependent, inferring the extent of carbonization without acknowledging this feedstock-dependency can be insufficient or biased. In consequence, it can lead to non-optimal modification or use of biochar in consecutive processes.

The biochar structure contains aromatic rings with different degree of aromatization, which is related to the overall carbonization. The aromaticity of biochar has been found to be strongly dependent on (i) feedstock-dependent features and (ii) production-dependent features [[5], [6], [7], [8], [9], [10]]. The specific influence of the feedstock-dependent features is complex and appears randomised. Nevertheless, some general trends are apparent from literature. Biochar derived from a lignin-rich feedstock (i.e. wood and its residues) tends to reach higher aromaticity, compared to biochar from mineral-rich feedstocks (i.e. crop residues and processed waste materials like manures and sewage sludge) obtained under the same processing conditions [[5], [6], [7], [8], [9], [10]].The impact of production-dependent parameters, especially the HTT in pyrolysis on the aromaticity and extent of charring is more comprehensible. It is well known that upon increasing the HTT, a progressive elimination of heteroatoms (through dehydration, decarbonylation and decarboxylation reactions) occurs [11], along with rearrangements (i.e. poly-condensation reactions) in the carbonaceous structure that promote the formation of (poly)aromatic clusters [8,12,13]. Moreover, an increase in temperature increases the degree of aromatic condensation (i.e. the cluster size and the purity of the aromatic structure) as observed through ¹³C NMR spectroscopy [8,14,15]. As a result, biochar obtained at higher HTT features particular levels in the aromaticity and degree of aromatic condensation which are not observed in biochar produced at a lower temperature [8]. Unfortunately, the ¹³C NMR spectroscopy analysis method, despite its accuracy and reliability, requires expensive instruments, which additionally are not straightforward to use. Therefore, relatively simple and low-cost biochar chemical characterization methods were pursued and introduced, whose role is to indirectly assess the carbonization level of biochar in a less accurate, yet less time-cost expensive manner.

The simplest and most frequently used ones are based on the elemental and proximate analysis, such as H/C molar ratio or fixed carbon content (FC) on a dry basis [16]. Considering that the most stable carbonaceous material is anthracite/graphite with a very well-developed structural organisation and whose H/C is very low and with a FC content close to 100 %, other carbonaceous materials can be ranked according to their carbonization level in relation to these reference materials. The R50 stability proxy is based on a very similar basis [17]. Another, relatively new method is the Edinburg stability tool (Æ), which assess the resistance to chemical oxidation of biochar C [18]. It assumes that the better-developed structure, i.e. a more aromatic char, is more resistant to mineralisation, hence more stable. More complex chemical indicators are the ones obtained via analytical pyrolysis (Py-GC/MS), such as the benzene to toluene ratio (B/T ratio). Analytical pyrolysis methods are based on the assumption that more recalcitrant carbonaceous structures release less oxygenated or branched aliphatic compounds, as these compounds should already have been released upon the actual char production process. As it can be noticed, all the mentioned biochar characterization methods are indirectly related with the carbonaceous material structural organisation (e.g. aromatization and the extent thereof).

Since changes in the degree of aromatic condensation can occur partially feedstock-independently, the HTT could be considered as a basic indicator of the extent of the biochar’s aromatization. Therefore, considering a large-scale production, it could be useful to biochar end-users, producers, and certifiers to know the actual temperature in which biomass was converted. The aim of this study is to create a simple-to-use correlation based on easy-to-measure properties of given biochar, which would allow for quick assessment of its HTT after production. For this purpose, this study assesses the feedstock-independent nature of various established biochar characterization methods described in literature via statistical tools like principal component analysis (PCA). Then, the characterization methods are checked in terms of their predictive power and reliability. This study provides a multilinear correlation between selected predictors and HTT. The obtained MLR model is then validated against various external datasets to assess its accuracy and usefulness.